An important process to the overall gasoline production in the world is the refining Fluid Catalytic Cracking (“FCC”) related processes. FCCs utilize very small particulate catalysts which are raised to very high temperatures and subsequently fluidized. These fluidized particles contact high molecular weight petroleum feeds and catalytically “crack” these larger hydrocarbon molecules to lower boiling products which are more valuable products. Most FCC processes contact heavy feed oils (such as vacuum gas oils, atmospheric gas oils, and often petroleum resids) with the fluidized catalysts typically with the goal to maximize naphtha production volumes.
In the FCC process these low-value, high boiling point hydrocarbon feedstocks are catalytically converted into more valuable products by contacting the feeds with fluidized catalyst particles in the process. In modern “short contact time” fluidized catalytic cracking (FCC) units, the hydrocarbon feedstocks are typically contacted with the fluidized catalyst particles in the riser section of the FCC reactor. The contacting between feed and catalyst is controlled according to the type of product desired. In catalytic cracking of the feed, reactor conditions such as temperature and contact time are controlled to maximize the products desired, such as naphthas, and minimize the formation of less desirable products such as light gases and coke.
The FCC naphthas derived from such processes are very valuable products as they are used as a component in final gasoline production. FCC naphthas can often account for about 50% or more of the overall “gasoline blending feedstock” in a refinery. Additionally FCC naphthas typically have a relatively high octane value as compared to “straight run” naphthas that are typically produced by a refinery's crude unit. This high octane value of the FCC naphthas is in large part due to the high olefin content of the FCC naphthas. As such, maximizing the total of production of FCC naphthas suitable for gasoline blending is of significant importance to any commercial refinery.
However, due to environmental regulations imposed within the last 10 to 15 years, most commercial gasolines have to meet a very low sulfur content specification of less than 30 ppmw sulfur. Most FCC naphthas cannot meet this low sulfur specification and must further undergo some type of hydrodesulfurization processing in order to meet these low sulfur specifications. An example of a preferred naphtha hydrodesulfurization processes is the SCANFINING® process which is licensed by the ExxonMobil Corporation. These processes utilize specialized catalysts and processes targeting desulfurization of naphthas to meet low sulfur gasoline specifications while retaining high octane values in the desulfurized naphtha products.
However, a problem exists in the art that problems can be experienced in many naphtha hydrodesulfurization processes due to equipment pluggage, catalyst bed pluggage and catalyst deactivation especially when treating cat naphthas. Typically, most cat naphthas are required to be sent for further catalytic hydrodesulfurization. This is due to their high sulfur content (usually well above 100 ppmw sulfur).
However, due to pluggage problems in the naphtha hydrodesulfurization (“HDS”) reactors and associated equipment when operating with certain (not all) cat naphthas, a present practice is to make a lighter boiling point end cuts on the cat naphtha fraction. That is, instead of making a full cut cat naphtha (say to a full 450° F., end point distillation), the refiner may, for instance, make a boiling point cat naphtha fractionation end cut at 400° F. While this may help alleviate the problems in the naphtha HDS reactor units, this presents a significant cut in the refinery's overall FCC gasoline production. In this case, these “cut” gasoline fractions typically have to be sent to lower value kerosene or distillate fuel products. This action results in a significant negative economic impact to the refinery.
What is needed in the industry is a low cost, low capital process for pretreating and hydrodesulfurizing FCC “cat” naphthas in order to eliminate these plugging problems or the alternative disadvantaged process of downgrading portions of the cat naphtha pool to lower value products. What is needed is a process for solving these problems while still obtaining a high-volume, low-sulfur, and high-octane naphtha blendstock pool for gasoline production.